(1) Field of the Invention
The present invention relates to an optical interference filter used for a display device such as a liquid crystal display or a plasma display, and more particularly to a technique to improve efficiency of color separation.
(2) Description of the Related Art
In recent years, a multilayer optical interference filter has been prevalent as a color filter that can offer a stable characteristic with respect to heat resistance and light resistance while withstanding chemicals such as acid and alkaline. The multilayer optical interference filter realizes the performance of separating colors including red, green, blue and so on by the interference effect of laminated films, which are formed by thin films of inorganic compounds being laminated on each other.
FIG. 10 shows the structure of such a conventional color filter as described above.
This color filter is formed such that Titanium Dioxide (TiO2) which is a high refractive index material and a Silicon Dioxide (SiO2) which is a low refractive index material are alternately laminated on each other on a transparent substrate, then the optical film thickness is adjusted so that lights of color elements including red, green and blue are selectively transmitted.
Specifically, in order to selectively transmit lights that each have red, green and blue light wavebands, the above-described color filter generally has a thin-film multilayer structure in which the optical film thickness is optimized for each of the red, green, and blue light wavebands. For example, the total film thickness of each color part of the thin film multilayer is as follows: red (R): 1.5 μm, green (G): 1.9 μm, blue (B): 1.1 μm.
Here, for the sake of convenience, the parts of the thin film multilayer that selectively transmit red, green and blue lights are each referred to as red transmission region, green transmission region and blue transmission region.
In the above-described thin film multilayer, the film thickness of the blue transmission region is smaller than the green transmission region and the red transmission region. This is because the wavelength of blue light is shorter than the wavelengths of green and red lights. The film thickness of the blue transmission region is set to be small in order to set the light having a shorter wavelength in the blue transmission region to be the peak of transmittance.
Also, the film thickness of the green transmission region is particularly large since the thin-film multilayer structure of the red transmission region is combined with that of the blue transmission region to obtain a spectral characteristic of transmitting only the green waveband light.
The above-described color filter, which is formed by thin films of inorganic compounds being laminated on each other, even when used as the color filter of a color liquid crystal projector, exhibits less deterioration caused by heat generated from the intense light source of the projector and ultraviolet rays contained in the light source.
However, as described above, the total film thicknesses of the red transmission region, green transmission region, and blue transmission region are red (R): 1.5 μm, green (G): 1.9 μm, blue (B): 1.1 μm. For example, there is a 0.8 μm difference in film thickness between the green transmission region and the blue transmission region, and there is a 0.4 μm difference in film thickness between the red transmission region and the blue transmission region. Therefore, light that has entered obliquely in a transmission region may also enter an adjacent transmission region.
In this case, a so-called “color mixing” in which two or all colors out of the three primary colors are mixed together occurs and the color separation efficiency deteriorates.
In order to suppress the color mixing caused by oblique light that is transmitted through two transmission regions whose filter characteristics are different from each other, a functional part that blocks light, namely, a black matrix may be arranged in the boundary region of the two adjacent transmission regions. However, the black matrix is generally arranged away from the boundary region in the thickness direction of the filter. Therefore, it is not effective to prevent the color mixing that occurs in the part that spans between the adjacent transmission regions.
When the black matrix is formed, a method that laminates a light shielding material such as chromium (Cr), nickel (Ni), or a metallic oxide is used. If the light shielding material is attempted to be formed in the boundary region, namely, the part that spans between the adjacent transmission regions, the conventional manufacturing facilities cannot be used as they are, thereby increasing cost. Also, from a technical standpoint, a secondary problem such as an increase of a distance between pixels may arise.